Welcome to Trouble Free Pool

We would like to welcome you to TFP, the webs leading Pool related website. We invite you to please Register to create a new account. Doing this will give you full access to the forums, where you will be able to receive personalized advice concerning your pool. We hope to see you soon!

Adding Acid Via The Skimmer

At the risk of stirring up a hornets nest, I would like to get some feedback on the long held belief that adding acid via the skimmer may damage some equipment with metal. In most cases this probably not a wise thing to do but for some pools under some situations could this method actually be beneficial? Let me explain why I think so using my situation as an example.

First, I have a Northstar pump which happens to use a synthetic impeller which is, as far as I know, not subject to corrosion. Some sort of polymer or fiberglass.

Second, the only other components in my setup which could be damaged by acid are the heater's heat exchanger and the swg.

Third, my fill water is quite high in PH, TA and CH. Over time CH builds up to a point where I basically have to do a refill about once a year. My CH starts off at 200 ppm but a year later is well over 600 ppm. Also, because of evaporation and the SWG, each week I add about 1/2 gallon of acid. And yes, I tried adding Borax but I think the fill water trumps any benefit Borax may have and I still have had to add the same amount of acid.

Fourth, by the time acid hits the metal equipment, I estimate that it is diluted to a ratio of about 40:1. The flow rate of my system is about 88 GPM and I can pour acid at a rate of about ½ gallon in 15 seconds or about 2 GPM, so my dilution ratio is higher than about 40:1. In addition, the low PH would only occur for the duration of the pour which is about 15 seconds.

So since I probably have a decent coating of calcium on the SWG plates and the heat exchanger, intuition would tell me that the low PH for 15 seconds would probably not cause any damage. In fact, the occasional dosing of acid would help keep calcium from building up on the plates of the SWG as well as the heat exchanger.

One more side benefit for a plaster pool is that there is no chance of acid pooling at the bottom of the pool that can occur even if poured near a return.

In addition, the SWG manual suggests using 2:1 water to acid solution for cleaning the cell once every three months for several minutes. So in terms of damage to the cell would 4 times a year at 2:1 for several minutes be better than 52 times a year at 40:1 for 15 seconds? Again, I would think more often at lower acid concentrations would be less destructive to the cell but that is just a hunch.

So my question to the chemists out there, would subjecting metal to 15 seconds of 40:1 acid dilution once a week cause any appreciable corrosion especially when there is a high calcium concentration in the water as well?

So am I missing something here or is this a viable method of adding acid? Maybe, just not for everyone.

We dummies are mildly offended that we were not invited to respond as well as the chemists!!

An invitation to the party is only a formality, anyway, so here goes:

I am in complete agreement with your assesment. I have neither a heater nor an SWG to consider in the problem so the benefit does not pertain to my pool. What I think is important, however, is the dilution examples you posted. At those ratios and considering the exposure time, I question any harm at all could come to any metals in the system. You could even use a slower pour rate for even more dilution if you chose and virtually eliminate any potential for harm.

That said, I still pour mine near a return. I happen to have a very high-power return located conveniently and it is normally pointed toward the surface. I use about a 1 gpm pour rate so the dilution is almost instantaneous and very effective. I can envision a pool where it would be more effective to dilute from the skimmer, tho, so I think some common sense will dictate how each person applies acid. I cannot comment on the potential benefit to the SWG but you may well be on to something. The exposure is s-o-o short-lived that I question whether it will have an affect.

Also, I think it comes down to what ever works for you. This is most certainly not for everyone but perhaps an option for those who would prefer this over a deep end pour and help keep the SWG clean at the same time.

I used to pour acid in a bucket of water and then add slowly around the perimeter.........until I got a solar cover. My PH climbs quickly so I add about .5 to 1 gal of acid weekly and now I find myself too lazy to remove the cover to add acid. So lately I have been adding it in my skimmer really slow and the pump on hi speed. The only concern I have is having metal in my pump some how.

How long do you think it takes the acid from going from the skimmer out to the return? I think pouring some acid, wait a few sec and pour a little more should be safe.

I believe that the only significant risk is to the heater, and that only if it uses a copper based heat exchange coil. The SWG should be fine even at much higher concentrations of acid. There is some question of the safety of the pump seal, I am not sure about that. If there is actual calcium buildup in the heater then you should also be fine. If there isn't calcium buildup in the heater, and I suspect that there wouldn't be, then there would be some small/tiny amount of damage to the heater each time. Muriatic acid is a very strong acid. The PH of the water when it got to the heater would actually be quite low. I wouldn't worry about doing that once, but doing it every week would have a cumulative effect that I would expect to shorten the life of the heater detectably.

I agree with these assessments including the details Jason just provided. The only thing I can add from a chemist perspective (even though I'm not really a chemist -- just interested in chemistry) is that a dilution ratio of 40:1 isn't that high when it comes to a really strong acid. Muriatic Acid (31.45% Hydrochloric Acid) has a pH of around -1.0 (yes that's a negative one since the scale is logarithmic). A dilution of 40:1 with pure unbuffered water would bring the pH up to +0.6 and my calculations show that the normal buffering of pool water with a TA of 100 and CYA of 30 would, with 40:1 dilution, result in nearly that same +0.6 pH. Essentially, 1% of the volume of Muritatic acid that you add wipes out the buffering and the other 99% then lowers the pH dramatically.

pH is a logarithmic scale, so the following shows the dilution rates for the acid in unbuffered water and in buffered water starting from a pH of 7.5 (with a TA of 100 and CYA of 30):

Basically, once the buffering runs out, the pH drops dramatically fast. The buffering of the water is fine for adding one gallon in 10,000 gallons, but runs out at around 2-1/4 gallons in 10,000 gallons and that's not counting any buffering loss from the faster outgassing of carbon dioxide that would occur as the pH is lowered.

What I do not know is the rate of corrosion, say of copper, when exposed to a pH of +0.6, but I suspect it's relatively rapid so that as John says a single exposure might be OK, but regular exposure might strip a small amount of metal each week and that could add up to be significant over months or years.

I have an A.B. (in the College of Letters and Science they don't call it a B.A.) from U.C. Berkeley with a field major in physical sciences since they didn't allow a double major in physics and chemistry since there were too many overlapping courses (mostly in mathematics) and the physics degree was in L&S while the chemistry degree was in the College of Chemistry. So I took courses that would be roughly equivalent to a physics and chemistry degree in total, but minus a few advanced courses in each (i.e. I picked the ones I was most interested in) since I still wanted to graduate in 4 years. I did not learn German, for example, which was pretty much required for a pure Chemistry degree. I had a 3.8 GPA (counting the 12 A+'s as 4.0). I also have an MBA from the same University.

I also had an interest in computer science, but found the entry "screening" courses to be killer "filters" I didn't want to bother with (i.e. staying up all night to get computer time at terminals and having to work to death to whittle down the limited number of slots they had at that time) so I picked up Knuth and other books and learned on my own. I then went to work as a programmer and a variety of subsequent positions in industries mostly working in software development and management which is what I still do today.

When this pool chemistry stuff came up for me a few years ago (when I had problems using Trichlor pucks), I dusted off my chemistry books and checked on the web to see if there was anything new and exciting I should know and then worked out the calculations which, though tedious (especially using an Excel spreadsheet), aren't based on anything more advanced than lower division undergraduate (or some A.P. high school) chemistry (equilibrium, ionic strength, ion pairs) and mathematics (simultaneous linear and non-linear equations, derivatives, Newton-Raphson) plus equilibrium constants for the chlorine/CYA compounds mostly determined back in 1973. It felt good to actually use some of that knowledge that I had learned and not really applied for so many years.

Don't forget that others on this forum also have chemistry backgrounds such as waterbear even though they also (like myself) may not be chemists or have a focussed chemistry degree. As waterbear has pointed out to me before, I'm good at theoretical chemistry and he's got real-world experience with chemistry knowledge so the combination is better than either alone. The same can be said for the others that post on this forum (including yourself) as everyone has their piece to contribute and it's all valuable. And we must remember Ben Powell who started this all at The Pool Forum since without his initial observations, persistence, and website with many outstanding contributors, we would probably not have the knowledge we have today.

Thanks for the background information (which is very impressive, by the way). It is a shame that Cal would not let you get the double major. I had a room mate many years ago who got something called an Engineering Physics degree, which combined the toughest courses from mechanical engineering and physics. I thought I worked hard on my electrical engineering courses, but this guys coursework and homework was literally 2-3 times as difficult as mine was.

And I "fondly" remember using the Newton-Raphson equations/methodology, or as I used jokingly refer to it as "Ralph Newtson".

And we must remember Ben Powell who started this all at The Pool Forum since without his initial observations, persistence, and website with many outstanding contributors, we would probably not have the knowledge we have today.

I fully agree. I am still in awe of Ben's ability to put forth a FC/CYA relationship when literally no one else seems to have either admitted it, or were able to put the relationship down in numbers.

For some reason, I find tinkering with the various aspects of the pool - chemical, plulmbing, electrical - to be more satisfying than my day job used to be.

Getting back on topic...
The biggest dangers from pouring acid into the skimmer (or putting trichlor in the skimmer) are to the pump seals and to filter cartridges if you have a cartridge filter. IMHO, you can ruin the cart faster than the seals but both will suffer damage in not that long a time if the process is a regular one.
Damage to pump seals is obvious, they start to leak but the damage to carts actually can take on two different appearances.
The obvious one is the degredation of the rubber end caps on the cart. They get gummy and can actually start to 'dissolve' and leave a residue throughout the system and plumbing. I experiened this one first hand when my pool was being built (thanks to an idiot pool builder who is now out of business) and I actually had to wipe out residue from the dissolved end caps from my skimmer! (Not to mention the mess it made inside my filter chamber!)
The second type of damage is a bit harder to spot but the usual symptom is that your filter pressure increases rapidly after freshly cleaning the cart. The reason for this is exactly the same reason that you do NOT want to acid wash a cart until you have degreased it first. The acid hardens the organics that collect on the cart into a cement like mass that effectivly ruins the cart and requires it to need replacement. This is documented on both the Unicel Filters and the Pleatco websites in their sections on proper cleaning of carts, btw.

Also, continued expose to acid will cause the polyester material the cart is made of to stretch and actually lessen it's filtering ability. This is why it is recommended to only acid wash a cart when it is needed (scale deposits are present after soaking n a degreaser).

Bottom line is this, pouring acid into your skimmer is risky at best and downright dangerours at worst!

Hope this info is useful.

Edit: Just read mas965's post (we must have posted at the same time!) and he brought up some excellent points about heaters, which totally slipped my mind! IMHO, the danger in order of most likely to least would be heater (with copper exchanger), cart, and then pump seals! If the heater has a titanium exchanger it might stand up for a bit longer to low pH conditions but it will still suffer damage over time. Also remember that besides damaging a copper heat exchanger you will end up with copper in the water and most likely will suffer the assciated staining of your pool suface and possibly some green hair! ( I have actually seen this occur in customer's pools. They had a period of low pH water and their copper levels shot through the roof! Usually they needed to replace the heater because the exchanger was shot at this point. Realize that, while this damage did not occur overnight it really only took a few weeks for it to occur.)

So it would seem that the heater is at the largest risk. Manufactures generally recommend that a CH level of at least 200 ppm be maintained in pools with a heater to avoid corrosion. They claim that a thin layer of calcium helps protect the heat exchanger. Although not a very thick layer, I would assume that this would help protect it during the short period of exposure.

In addition, given the lifespan of the heater is probably not much more than 10 years, at 15 seconds per week, that is only a little over 2 hours of soak time in low PH water. I have a hard time believing that this would cause any appreciable damage to a pipe even if it was copper. I suspect that the long term exposure to water, salt and chlorine would have more damanging effects since it would be constant over the same time period.

I feel another bucket test comming on.

[EDIT] Just read waterbears response which he brings up a some good points.

The theory about forming a thin layer of calcium carbonate scale as protection on metal surfaces seems to not have good scientific backing. In practice from what I could find reading about it, such films are uneven at best and it's very difficult to control the scale growth -- it just doesn't form a small layer and then stop growing. Instead, it's possible that having the water near saturation helps somewhat interfere with the corrosion rate, but even that is disputed by some. Virtually everyone says that pH is the biggest factor in corrosion by far though at normal pH the level of oxidizer is the next most important factor and conductivity is important as well -- so your points about chlorine and salt levels are well taken.

Just remember that water used to be chlorinated to around 1 ppm FC (now monochloramine tends to be used instead) though the residual was sometimes lower by the time it reached the home and this was without CYA and that such water was in copper pipes in most homes (today, around 1 ppm of monochloramine is often used). The salt level was very low so conductivity was low, but the pH was usually higher closer to 8.0 or at least 7.5. The dissolved oxygen amount may be low as well and that is another factor that is often quoted. In many municipal water districts, the water is soft to moderate so calcium hardness is low compared to pools (50 ppm) and TA isn't high (80 ppm) so the saturation index is very negative (corrosive, at least to cement/grout surfaces). Anyway, copper pipe wasn't corroding, or at least not very quickly, in everyone's home.

Traditional drinking water delivery systems do form a protective layer on the inside of the pipes. This was dramatically illustrated around here a couple of years ago when they switched from chlorine to chloramine and suddenly half the city had dangerous lead levels in their water. The problem was solved by adding orthophosphate to the water to inhibit the corrosion. Eventually they will also replace the lead pipes. The research they did showed that free chlorine actually combined with the lead to form a protective layer. So it wasn't calcium scale, but something else that was protecting the pipes.

The water delivery chemistry is quite complex. They had to run about 100 different water treatment combinations through actual pipes for weeks to see which treatment option would solve the lead corrosion problem without introducing new problems either for the lead pipe or other pipes commonly used in the area. Basically, it wasn't possible to predict what the corrosion would be given the water chemistry, they had to actually try many different combinations and see what happened.

Likewise, we know that muriatic acid attacks copper, but figuring out the rate is problematic. Quite a number of factors can affect the corrosion rate. There is no table of rates you can look the reaction up in and say "erosion of 1mm every 37 hours" or anything like that. All we can do is make very rough estimates, knowing that real world conditions will be different.

There are three examples I know of are: First, if you put copper wire in full strength muriatic acid and leave it for a week the copper wire is obviously corroded, but not eroded all the way through. Second, there are known cases of several weeks of low PH destroying copper heat exchange coils. In those cases the PH was known to be below 7, but it isn't clear how much lower. And third, we know that some people add muriatic acid through the skimmer and their heaters do not fail right then and there.

None of those examples provide any clear evidence for the question of short acid exposures. But the first two suggest to me that we are no farther than two orders of magnitude from dangerous levels of corrosion. Now if it is really two orders of magnitude away, then you might take a couple of weeks off the life of your heater and no harm done. But given all the other things that might affect the corrosion rate and the level of uncertainty involved it isn't something I would want to try or recommend.

Thanks Jason. Good info. Real world testing is the only way things are known for certain. My discussion was only with copper piping, not lead pipes, [EDIT] and specifically about calcium carbonate films [END-EDIT] but the points are all still valid -- the chemistry of corrosion is complex and real-world tests are the only true way of determining what will really happen.

The anecdotal evidence I have seen on pool forums regarding copper heat exchanger corrosion seemed to indicate that a sustained pH of 3.5 - 4.5 over several months up to a full season would lead to a complete breakdown (i.e. getting thin enough such that the heat and pressure caused a pipe burst, cracked, had a leak, or the PB said the exchanger was shot). Every case that was reported (and there were only a handful -- perhaps 3 or 4) had the TA test turn red immediately which implies a pH below 4.5 (all of these cases were Trichlor users that didn't test and maintain the pH in their pools). Based on the amount of base that was needed to make the pH rise to a certain level, I estimated the actual pH to be in the 3.5 to 4.5 range for all of the pools that reported this problem. On the other hand, there were more pools that had the pH be that low that didn't have obvious problems, but they were for shorter duration, typically a month or less.

As Mark pointed out, a beaker test using real pool water with Muriatic Acid added in equivalent 40:1 dilution would give a better idea of what to expect. Trying this with 3000 ppm salt would also be interesting to see what difference that makes.

By the way, it is interesting to note that Ferric Chloride as an oxidizer will etch copper rather quickly, though I suspect it's concentration is very high -- probably like using concentrated chlorine and certainly not like 1 ppm.

The SWG doesn't seem to be too vulnerable since it is usually cleaned with much higher acid concetrations.

For the heat exchanger and rubber seals, my original thought was that the exposure was so short, the equivalent of 2 hours over 10 years, that it probably would not make that much difference. This may be true for the cartridge as well since the concentration is much lower than normal cartridge cleaning. I guess the only way to know for sure is to run an experiment.

The info here reemphasizes that pool water chemistry is not as precise and neat as we'd like. Fretting over our FC @ 3ppm or 4ppm is simply not that critical. Shock values of 16ppm or 18ppm doesn't make much difference. Alk of 100 or 110 is not likely to cause much of an issue.

What IS important is that we know generally safe ranges for our water chemistry and that we understand how to adjust our water to stay within those ranges.

On one of the test pools we have, some tests we done needed sharp change in pH of the pool water. We need to drop pH 8.2 down to 6.9 or about. The only way to do it quickly is to feed acid via skimmer. Some time we need to dump about 40 Lt of HCL in one go (under one minute). Based on the test results and inspection of equipment used, I can say that if feeding acid slowly in to the skimmer, given that water pump flows 300-400L/m there is no adverse effect as acid get diluted. It is also recommended to have the filter in BY-PASS mode, otherwise you will flash a lot of dirt back in the pool.

Dumping acid in the skimmer in large amounts, like 20Lt in 20 seconds had adverse effect on the pump SS shaft and damaged the seals. This is not over single event, but over the period of time with repetitive abuse.

Titanium,
I work for chlorinators manufacturer. We perform a lot of R&D, some of the tests are long shoot projects that end users will see coming out in 5-6 years from now.
Acid damp tests are normally used to test performance of chemical sensors and as well as electrode "cell". They are also used to evaluate some secondary data such as calculated pH change VS actual pH change in short time bracket... Thinks like that....